Carburetors for internal combustion engines typically include venturi passages for the passage of air from the atmosphere to the engine, with the venturi passages being used to create a zone of low pressure that induces flow of fuel into the stream of air. Butterfly valves are used to variably constrict, and therefore control, the volume of air passed through the bores of the carburetor.
In addition, most carburetors include other passages that are not controlled by the butterfly valves but are controlled by needle valves or other constriction means that can be set and are not variable during the normal operation of the carburetor. A purpose of one of these additional fuel passages is to continue to pass smaller amounts of fuel and air through the carburetor to the engine so as to allow the engine to idle at slow speeds.
Typically, in a four barrel carburetor there will be four idle passages, and each idle passage will have its own needle valve or other means for constricting the flow of fuel and air passing there through.
A typical problem with the prior art designs is that it is difficult to adjust the volume of fuel that moves through each idle passage. Typically, the technician will attempt to adjust all of the valves so that the flow of fuel and air through all of the idle passages is equal. This tends to balance the volume of fuel and air to each cylinder of the engine. If the valves that control the flow of fuel throughout the idle passages are not equally adjusted, some cylinders will receive more fuel than others, resulting in non-uniform or “rough” engine performance during idling.